The invention relates to a magnetron cathode sputtering system comprising in an envelope an anode and a flat cathode including a target, which is connected against a backing plate behind which means are provided to form a magnetic field near the target in the envelope. The target is a plate of material to be sputtered and is connected against a backing plate, which is a supporting plate for the target. Such systems are frequently used for providing thin films of material on substrates. During operation of the magnetron cathode sputtering system the substrate is placed in the envelope, the envelope being filled with a gas. By applying a sufficiently high potential between the anode and the cathode, a gas discharge is formed in which gas ions impinge on the target at high velocity and liberate the material therefrom (mainly atoms). This sputtered material is then captured by the substrate. By providing magnetic field lines near the target, which trap electrons, more ions are formed and hence the sputtering process is intensified. As a result of this the desired layer is formed more rapidly on the substrate. In such systems the target must be cooled to prevent the target from melting and to prevent the target from radiating too much heat. A magnetron cathode sputtering system of the kind mentioned in the opening paragraph is known from U.S. Pat. No. 4,204,936. The target according to the invention described in said Patent Specification is clamped against the backing plate by means of one or more magnets, the magnets cooperating with magnets present behind the backing plate. A disadvantage of such a device is that the clamping force per surface unit is low. Moreover the magnetic field for the target is adversely influenced. Such sputtering systems are used for the manufacture of optical information carriers in which a substrate, consisting of a thin plastic or glass disc in which the information is present in the form of a large number of pits, is covered with a thin metal film. In some devices the targets are connected to the backing plate via a soldered joint to produce a good mechanical and thermal contact. However, if the target is soldered to the backing plate it is very difficult to separate the target and backing plate from each other after sputtering in order that the backing plate can be used again. Moreover it is difficult in the case of large dimensions of the two plates to realize a uniformly soldered joint throughout the surface. British Pat. No. 1,311,042 discloses a high-frequency cathode sputtering system in which a target is secured against the backing plate by means of a screwed clamping ring. A material readily conducting thermal energy is provided between the target and the backing plate, for example a liquid metal or a metal paste. With such cooling it has also proved impossible to produce a uniform thermal contact between the target and the backing plate which is necessary for a magnetron cathode sputtering system. Moreover, seals must be provided to prevent the liquid metal from being disposed in the envelope.